An electric current is a stream of charged particles, such as
electrons or ions, moving through an electrical conductor or space.
It is measured as the net rate of flow of electric charge past a region.
Electric current is denoted with a symbol I
and is measured in amperes using a device called
an ammeter. The ampere is the flow of electric charge across a surface at the rate of one coulomb
per second.
Materials
There are three types of materials regarding their reaction to an electric current:
conductors,
insulators, and semiconductors.
A conductor is a material that allows the free flow of charges (electrical current)
in one or more directions. The best-known conductors are metals such as silver, gold,
and copper.
An insulator is a material in which the electron
does not flow freely.
Minimal electric current will flow through it under the influence of an electric field.
Insulators are used in electrical equipment to support and separate electrical
conductors without allowing current through themselves. An insulating material
used in bulk to wrap electrical cables or other equipment is called insulation.
Good insulators are rubber, glass, and porcelain.
A semiconductor material has an electrical
conductivity value falling between that of a conductor
and an insulator. Its resistivity falls as its temperature rises; metals
behave the opposite. Its conducting properties may be altered in useful ways
by introducing impurities ("doping") into the crystal structure.
Semiconductor devices can display a range of useful properties, such as passing
current more easily in one direction than the other, showing variable resistance,
and sensitivity to light or heat. Because the electrical properties
of a semiconductor material can be modified by doping, or by the application
of electrical fields or light, devices made from semiconductors can be used
for amplification, switching, and energy conversion. The most common semiconducting materials are silicon and germanium.
The source of current
An electric current will appear if there is a difference of electrons between two poles
of conducting material, or, as it is also called, the potential difference
(which is the same as voltage).
There are more electrons on the right pole of the conductor than on its left. Thus the right pole has
a negative charge. The left pole, which has more protons than electrons, has a positive charge.
Since opposite charges attract each other, electrons from the right pole begin to flow to the
left, creating electric current until the number of protons and electrons on both poles are
equal, and equilibrium is reached. At that moment, the current ceases to exist.
In a conductive material, the moving charged particles that constitute
the electric current are called charge carriers. In metals, which make up
the wires and other conductors in most electrical circuits, the positively
charged atomic nuclei of the atoms are held in a fixed position, and
the negatively charged electrons are the charge carriers, free to move
about in the metal. In other materials, notably the semiconductors,
the charge carriers can be positive or negative, depending on the dopant used.
Types of current
In alternating current systems, the movement of electric charge periodically reverses direction.
Alternating current is the form of electric power most commonly delivered to businesses and residences. The usual
waveform of an alternating current power circuit is a sine wave, though certain applications use alternative waveforms,
such as triangular or square waves. Audio and radio signals carried on electrical wires are also
examples of alternating current.
In contrast, direct current refers to a system in which
electric charge movement occurs in only one direction
(sometimes called unidirectional flow). Direct current is produced by sources
such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo
type. Alternating current can also be converted to direct current through use of a rectifier.
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